Evaluating chest-worn light logger adherence: development and application of a wear/non-wear model.

Light exposure plays an important role in overall health because it entrains circadian rhythms. Recent technological advances in wearable light loggers allow measurement daily light exposure habits. Using a chest-worn light logger, our goal was to (1) develop methodology for differentiating adherent versus non-adherent use, and (2) define differences in lighting intensity in indoor and outdoor environments, to improve data reliability in future clinical studies using this technology. Four testers used a 10-channel chest worn light logging device under different conditions of wear and non-wear (experiment 1), and another tester made measurements with the light logger across a variety of indoor and outdoor lighting environments (experiment 2). In experiment 1, measurements from the light logger (photopic illuminance, device orientation, accelerometer data, time of day) were used to train models to differentiate wear from non-wear and correct nighttime placement. The most accurate model was then tested with the remaining data from testers (T1 - T4). This model was then applied to 20 adolescents and young adults with migraine who wore the light logger device for one week. In experiment 2, measurements of photopic illuminance and melanopic equivalent daytime illuminance (mEDI) of darker indoor versus brighter outdoor lighting environments were compared to identify the optimal indoor/outdoor distinction point for the chest-worn light logger. Movement, device orientation, light, and time-of-day used as predictors in a random forest model had excellent differentiation between wear, non-wear and nighttime use (overall accuracy 0.95), and retained good-to-excellent differentiation when applied to the test dataset (overall accuracy 0.76). When this model was applied to 20 participants with migraine, we found that 92.1% of participant-days and 77.9% of participant-nights demonstrated at least 80% appropriate use. Applying the wear/non-wear model to this dataset led to modest changes in light exposure measurements but did not change conclusions of our prior study. For differentiating indoor and outdoor lighting environments, the optimal cut-point was 442 lx for photopic illuminance, and 412 lx for mEDI. We demonstrate that internal measurements from a chest-worn light logging device can reliably differentiate wear from non-wear. We also found that the optimal cut-off to differentiate indoor and outdoor lighting environments was lower than 1,000 lx cut-offs traditionally used to define bright light conditions. These findings can be used to improve data reliability in studies of everyday light exposure in clinical populations using chest-worn light loggers.